In recent years, as is well known, there have been increasingly stronger calls for reduction of the weight of the hoses used for fluorocarbon gases etc. and the hoses used for transporting fuels using low molecular weight hydrocarbon compounds.
However, use of hoses which have been processed to prevent permeation of the refrigerant or fuel as the hoses for transport of refrigerants or for transport of fuels is important in terms of environmental protection. The important properties which the materials used for such applications should have are a superior gas barrier property and moisture barrier property with respect to Freon gas, hydrocarbon gas, etc., an excellent flexibility, a superior heat resistance, and a high rate of retention of physical properties when heated.
For example, in the above various applications in automobile air-conditioners etc. in the past chlorofluorocarbons (CFCs), which is one type of fluorocarbon gas, are used as the refrigerant. However, as is well known in the art, fluorocarbons are problematical in that they cause destruction of the ozone layer, global warming, and other destruction of the environment. Therefore, use of certain types of fluorocarbons has been prohibited and alternative fluorocarbons such as hydrofluorocarbons (HFCs) not containing chlorine have come into use. These alternative fluorocarbons are less destructive to the ozone layer, but remain problematical in terms of global warming, and therefore, hoses for transport of refrigerants are still required to have a superior gas barrier property to prevent the refrigerant gas from being discharged to the outside.
On the other hand, if the moisture barrier property of hoses is low, moisture will pass through the hoses into the refrigerant and causes freezing. The ice will become a cause of breakdowns in the apparatus such as ending up breaking the compressor etc. Further, if a hose does not have excellent flexibility, there will be the problem that it will not be able to absorb the vibration of the apparatus, noise, etc. Further, it will not be able to be freely laid in the apparatus or between apparatuses. Further, if a rigid hose is forcibly bent to install it, problems will occur in the durability of the hose.
In the past, as a low permeability hose, a hose of an inner tube made of nitrile rubber (NBR) and an outer cover made of chloroprene rubber (CR) is used, but in recent years this has been shifting to hoses which satisfy the above important properties, that is, low permeability hoses which have an inner tube of a double layer construction and have as the inner layer of the inner tube a polyamide thermoplastic resin layer superior in gas barrier property.
The properties sought for such low permeability hoses will be explained in more detail below:
(1) Reduction of Permeation of, for example, Fluorocarbon Gases, Hydrocarbon Gases
Low permeability hoses have been developed for the purpose preventing environmental pollution and destruction of the ozone layer caused by dispersion into the air of gas permeating from the inside of hoses by basically reducing the amount of leakage of gas of conventional rubber hoses (for example, hoses having NBR inner tubes and CR outer covers) and in particular have been developed for elimination of maintenance, that is, making the replacement cycle for gas 10 years in the case of hoses for transporting refrigerants.
The leakage amount of conventional rubber hose had in general been 20 to 25 gf/m/72 hr (at 100.degree. C.) (gas leakage for 72 hours) and the refrigerant replacement cycle had been about two years. Therefore, to eliminate maintenance for 10 years, it is necessary to decrease the amount of gas leakage to 5 gf/m/72 hr (at 100.degree. C.) or less. Therefore, even with the above HFC, it is necessary to achieve the above values in order to eliminate the maintenance.
Further, the refrigerant used in the past had mainly been dichlorodifluoromethane (hereinafter, referred to as CFC 12), but in recent years, the less ozone layer destructive trifluoromonofluoroethane (hereinafter referred to as HFC134a) has appeared as one alternative. Therefore, even if using HFC134a as a refrigerant, due to the above-mentioned reasons, it is necessary to decrease the amount of gas leakage to 5 gf/m/72 hours (100.degree. C.) or less.
(2) Flexibility of Hoses
Low permeability hoses are meant for the transport of refrigerant, fuel, etc., but at the same time play important roles in absorbing the vibration of the portions where they are connected, for example, the compressors and coolers, and therefore, flexibility is also requested. Conventional rubber hoses have had the flexibility for achieving this purpose, therefore hoses which have been improved in gas barrier property as well have been required to have a flexibility equivalent to that of conventional rubber hoses, that is, a bending force or flexural stress of not more than 3.0 kgf, preferably not more than 2.0 kgf.
(3) Prevention of Leakage at Metal Parts
A low permeability hose, for example, connects between the above compressor, cooler, and other apparatuses by affixing the two ends of the hose to them by metal fittings etc. so as to secure the connections to the apparatuses and to prevent leakage of the refrigerant etc. If the rate of retention of physical properties of the hoses at the connection parts when heating is low, the initial stress applied to the connection parts will be reduced by the heat during the use thereof causing a rapid decline in the residual stress and leakage of the refrigerant etc. from the connection parts. Therefore a low permeability hose is required to have a high rate of retention of stress at the time of heating.
As such a low permeability hose, there is known a low permeability hose having as an inner layer of the inner tube a polyamide thermoplastic resin layer and forming the outer layer of the inner tube and the outer cover by rubber. This hose satisfies the above properties and is superior functionally too, but requires a vulcanization process and has other problems resulting in high manufacturing costs.
As a hose for solving this problem, there has been proposed a low permeability hose having an inner tube of a single tube formed by a polyamide thermoplastic resin or other material with a superior gas barrier property and having an outer cover formed by a fluoro resin or other thermoplastic resin material superior in moisture barrier property which has few manufacturing steps and does not require cross-linking, and therefore, is low in manufacturing cost, but this hose is poor in flexibility and further has a low resistance to softening when heated, and therefore suffers from leakage at connections with metal and cannot be put to practical use.
To solve this problem, there is known a hose using a thermoplastic elastomer composed of a thermoplastic resin such as a polyolefin thermoplastic resin, polyvinyl chloride thermoplastic resin, polyamide thermoplastic resin, polyester thermoplastic resin, in which a cross-linked rubber has been dispersed (see Japanese Unexamined Patent Publication (Kokai) No. 6-64102). However, this thermoplastic elastomer cannot easily provide the desired properties in terms of the balance of the gas barrier property and flexibility, and therefore, cannot easily be used in practice for the desired hose applications.
In this way, there are not now known any low permeability hoses having the desired properties and functions and low in manufacturing cost.
A composition composed of thermoplastic resin/thermoplastic resin blends such as a high density polyethylene thermoplastic resin and nylon 6 or nylon 66 (HDPE/PA6,66), polyethylene terephthalate and aromatic nylon (PET/MDX6), polyethylene terephthalate and vinylalcohol-ethylene copolymer (PET/EVOH), which, by molding, results in one of the thermoplastic resins forming a layer and thereby providing a double layer to give a low gas permeation performance (gas barrier performance) and a process for its production are already known from Isao Hata: Polymers, 40(4), p. 244 (1991) etc. Further, the technique of using a rubber/thermoplastic resin blend composed of a specific modified butyl rubber/thermoplastic resin for a material for a low permeability hose was also already proposed by the present inventors (Japanese Patent Application No. 7-286168), but this material, while superior in flexibility, did not have a sufficient gas barrier property.
That is, the material for forming the gas barrier layer used for the inner tube of a hose etc. is required to have flexibility and a gas barrier property, but no material has yet be presented which has both of these properties.
A thermoplastic elastomer composition which is composed of a thermoplastic resin component as a continuous phase and an elastomer component as a dispersed phase and in which at least part of the elastomer component is cross-linked (vulcanized), has the rubber elasticity performance derived from the elastomer component which has generally been cross-linked in the past, and, due to the thermoplastic resin component forming the continuous phase, can be thermoplastically molded at a high temperature where it melts and becomes fluid, it is known.
That is, a thermoplastic elastomer composition having this dispersed structure has the characteristic of enabling processing by processing techniques similar to those of plastics while maintaining the properties of a vulcanized rubber.
Therefore, the above elastomer composition has the following basic advantages compared with vulcanized rubber:
(1) The vulcanization process is not required. PA1 (2) The recycle of the products and the scrap produced during the processing are possible. PA1 (3) The lighten in the weight is possible. PA1 mixing 15 to 80 parts by weight of a polyamide thermoplastic resin (component A) and 20 to 85 parts by weight of a rubber composition (component B) containing a copolymer rubber containing a copolymer rubber composed of a copolymer of a C.sub.4 to C.sub.7 isomonoolefin and p-alkylstyrene, where a part of the p-alkylstyrene unit portion has a halogen atom, at least at the melting temperature of the above component A, then PA1 adding a cross-linking agent during the mixing to cross-link the above component B and thereby give a thermoplastic elastomer composition wherein at least a part of the above component B is dispersed in the above component A and at least a part of a dispersed phase is cross-linked. PA1 .PHI..sub.m : volume fraction of thermoplastic resin component (A) PA1 a=-0.0518, b=0.90, and where the elastomer component (B) is the discontinuous phase and the thermoplastic resin component (A) is the continuous phase. PA1 .PHI..sub.B : volume fraction of functional thermoplastic resin component PA1 .eta..sub.A : melt viscosity of thermoplastic elastomer composition component at time of mixing PA1 .eta..sub.B : melt viscosity of functional thermoplastic resin component at time of mixing, and the volume fraction with respect to the total of (B) and (A) is controlled to 1 to 40%. PA1 preparing a thermoplastic elastomer composition (A) composed of a thermoplastic resin component constituting a continuous phase and an elastomer component constituting a dispersed phase in a ratio of weight of the latter to the former of 10/90 to 80/20, and then PA1 mixing into this an insoluble functional thermoplastic resin component (B) having a difference of the solubility parameter (.DELTA.SP value) with respect to the thermoplastic resin component of (A) more than 1.0 under conditions satisfying the following formula (4): and ##EQU5## where, .PHI..sub.A : volume fraction of thermoplastic elastomer composition component PA1 .PHI..sub.B : volume fraction of functional thermoplastic resin component PA1 .eta..sub.A : melt viscosity of thermoplastic elastomer composition component at time of mixing PA1 .eta..sub.B : melt viscosity of functional thermoplastic resin component at time of mixing
Among these, in particular, a thermoplastic elastomer composition where part or all of the elastomer component forming the dispersed phase is cross-linked (vulcanized) with the thermoplastic resin forming the continuous phase during the kneading (or mixing), that is, is dynamically cross-linked (vulcanized), can in particular give a product superior in the mechanical physical properties of a rubber elastomer, resistance to compression set, and resistance to oil and can be used, instead of conventional rubber, for auto parts, building materials, medical equipments, general industrial materials, etc.
The above thermoplastic elastomer composition for a low permeability hose use, when used for the hose inner tube or part of the outer cover, has an excellent gas barrier property and flexibility as well and further the lightness of weight can be extremely improved as well, but in the same way, with this thermoplastic elastomer alone, there is a problem of insufficient adhesiveness in the case of making a double layer structure with a reinforcing layer or if necessary another rubber material.
That is, as explained above, a thermoplastic elastomer composition where a thermoplastic resin component is used as the continuous phase and an elastomer component is used as the dispersed phase wherein the thermoplastic elastomer composition has rubber elasticity, is flexible, and can be thermoplastically processed and, further, enables the control of the air permeability or gas permeability is known. Further, a thermoplastic resin composition which can have formed integrally inside it a layer having a gas barrier property is known as well. Still, a thermoplastic elastomer composition having a rubber elasticity can be thermoplastically processed, and which can have formed integrally outside with a layer having the functions necessary for bonding has still not been known before.